In a comment on iPhonewzealand.co.nz, a user has pointed out that changes to tethering have been locked out in iPhone firmware 3.1. I imgaine this is at the behest of the big carriers like AT&T, in order to stop people making changes to their tethering APN to get around plan limits.
The unfortunate side effect is that it prevents users in New Zealand from modifying tethering to work on the XT network. iPhone calling and browsing still works, but tethering is currently broken.
There is a suggestion that small changes to the profile (ipcc) file will fix the issue. Check back for updates.